Adrenergic Receptor Agonists' Modulation of Dopaminergic and Non-dopaminergic Neurons in the Ventral Tegmental Area

The ventral tegmental area (VTA) neuronal population consists of dopaminergic (DAergic) and non-DAergic neurons (mainly GABAergic), the activity of which is intertwined with VTA behavioral functions. Both DAergic and GABAergic neurons in the VTA have been shown to express adrenergic receptors (ARs) and respond to AR stimulation. The aim of the present study was to demonstrate the effects of selective AR agonists on DAergic and non-DAergic neuronal activity in the central and lateral parts of the VTA using in vivo electrophysiological recording combined with microiontophoretic drug application in anaesthetized rats. Administration of phenylephrine, a selective α₁-AR agonist, while having an inhibitory effect on putative DAergic neurons (11% decrease in firing rate), induced a clear excitatory effect (59% increase in firing rate) on putative non-DAergic neurons. In contrast, application of clonidine, a selective α₂-AR agonist, or isoprenaline, a selective β-adrenergic receptor agonist, did not change the firing rate of either DAergic or non-DAergic neurons but influenced the firing pattern of non-DAergic cells only. Our results suggest that noradrenaline modulates activity of VTA neurons in vivo primarily via α₁, but also via β- and α₂-AR to a lesser extent. Furthermore, we show that α₁-AR activation has contrasting effects on putative DAergic and non-DAergic neurons. We hypothesize that the phenylephrine-induced inhibition of putative DAergic neurons results from activation of GABAergic terminals present at the site of drug application. Such a mechanism is further supported by the observed α₁-AR-induced excitation of putative GABAergic VTA neurons.